Recently, in magnetic recording-reproducing systems for computer data, thin-film magnetic heads have been practically used. Since the thin-film magnetic heads can be easily down-sized and applied to multi-track heads, they are widely used for multi-track linear recording systems, particularly, employing magnetic tapes as computer data recording media. The down-sized thin-film magnetic head is of value for increasing the track density to improve the recording efficiency, and consequently enables the magnetic tape to record the data in an increased density. Further, the multi-tracking system can increase the data transfer rate.
The thin-film magnetic heads are roughly categorized into two types, namely, magnetic inductive head (which responds to time-dependent variation of magnetic flux) and magnetoresistive head (which responds to scale of magnetic flux). Since the inductive head generally has a flat structure, its head coil consists of a relatively small number of turns. Accordingly, the inductive head can not give large magnetism, and hence it hardly gives high reproduction output. For this reason, the magneto-resistive head (MR head) giving high output power is used for reproduction, while the inductive head is used as a recording head. Those magnetic heads are generally incorporated in a unified system in which the linear recording method is adopted to perform rapid data transfer.
The specification of a magnetic tape for computer data recording is specifically determined according to the magnetic recording-reproducing system equipped with a MR head. For example, magnetic tapes to be used in IBM systems of 3480, 3490, 3590 and 3570 are known and commercially available. Each of those magnetic tapes has a basic structure in which a single thick magnetic layer (thickness: approx. 2.0 to 3.0 .mu.m) comprising a ferromagnetic powder and a binder is provided on a flexible support. In general, in order to ensure good running durability and to inhibit wrong winding, the magnetic tape for computer data recording further comprises a back-coating layer on the reverse side (i.e., side on which the magnetic layer is not provided) of the support.
It is a problem that the magnetic tape having the single thick magnetic layer can not store a great deal of data. To solve this problem, a magnetic tape having a upper magnetic layer and a lower essentially non-magnetic layer is proposed [Japanese patent provisional Publication No. 8(1996)-227517, which corresponds to Y. Kakuishi et al., U.S. patent application Ser. No. 08/602,567 filed Feb. 14, 1996]. In the proposed tape, a lower non-magnetic layer comprising a non-magnetic inorganic powder dispersed in a binder and an upper magnetic layer comprising a ferromagnetic powder dispersed in a binder are superposed and provided on a non-magnetic support. Since the upper magnetic layer can be made thin, output depression caused by thickness loss can be avoided in the proposed tape. Further, since the data recording density can be increased, the magnetic tape having the upper magnetic layer and the lower non-magnetic layer can store the computer data much more than that having the single thick magnetic layer. According to the above-identified publication, the upper magnetic layer has a thickness of 0.05 to 1.0 .mu.m, preferably 0.05 to 0.8 .mu.m. The magnetic tape for computer data recording described in the publication comprises a support of polyethylene terephthalate (thickness: 10 .mu.m). On one side of the support, a non-magnetic layer (thickness: 2.7 .mu.m) and a magnetic layer (thickness: 0.3 .mu.m) are superposed in this order. The magnetic layer contains a ferromagnetic metal powder having a coercive force (Hc) of 1,800 Oe.
The inventors have studied about adaptability of magnetic tapes to MR heads which is incorporated in the magnetic recording-reproducing systems, and found that the known magnetic tapes such as the above-described tapes are not satisfactory suitable for the system equipped with the MR head, particularly, for recording computer data. A further study of the inventors has revealed the following facts.
When the magnetic tape having a relatively thick magnetic layer (thickness: 0.3 .mu.m) is applied to the system, the reproduced output power is often so high that it saturates the MR head since the magnetic layer has a strong magnetic flux. The saturated MR head reproduces signals in distorted waveforms, and consequently gives an insufficient S/N value to increase the error rate. Further, although a high recording density is generally given by a sharp reproduction waveform (i.e., solitary reproduced inverse waveform) having a small half width, the thick magnetic layer enlarges the half width to lower the recording density. On the other hand, a magnetic tape having a very thin magnetic layer (thickness: 0.05 .mu.m) also distorts the reproduced waveform to give an insufficient S/N value. Further, the thin magnetic layer often lowers the reproduction output power.
It is an object of the present invention to provide a magnetic tape which can be applied to magnetic recording-reproducing system with a magnetoresistive reproducing head (MR head) suitable to exhibit excellent electro-magnetic characteristics. In more detail, the magnetic tape to be provided by the invention should cause few errors, accomplish a rapid data transfer rate, and record data of high densely when it is employed in a magnetic recording-reproducing system equipped with a magneto-resistive reproducing head.